Automotive air conditioning compressors are typically piston machines, in which reciprocating pistons within cylinder bores pull in refrigerant from a low pressure cavity on the back stroke, and drive it out to a high pressure discharge cavity on the up stroke. Reverse flow into the cylinder bores from the discharge cavity is prevented by one way discharge valves. Most often, the discharge valve uses a thin, resilient, elongated reed that is either riveted to the flat surface of a thicker, disk-shaped valve plate, or which is lanced integrally out of a thin metal sheet that is sandwiched to the surface of the valve plate. The valve plate separates the cylinder bores from the various refrigerant cavities. The front of the reed covers a refrigerant port through the plate, while the back end of the reed acts as a hinge. The reed bends up about the hinge, away from the surface of the plate, to allow flow across the port in only the desired direction, and snaps back down against the valve plate to prevent reverse flow. In this way, the assembly of reed and valve plate maintains the desired pressure differentials in a simple, passively responsive fashion.
While simple, the typical reed valve assembly does have some inherent limitations. Since the metal reed snaps up and down against the metal valve plate, it can cause noise. The reed also requires a metal stop to limit its upward bending, contact with which can cause noise. Since the reed bends with every piston stroke, it is stressed accordingly, to a degree that roughly corresponds to the height that it lifts from the surface of the valve plate while opening. The reed lift height must be great enough that flow across the port, which must pass around the sides of the reed, is not limited. In order to create enough reed lift height away from the valve plate, a minimal reed length is necessary, since a short reed will be inherently stiff. Moreover, the lift height varies along the length of the reed, being greatest at the front end, and less toward the back. Since the port has a finite diameter, the reed does not lift away from the port by a constant height, and flow will be more restricted through that area of the port that is closest to the hinge point. This requires that the lift height at the front of the reed be greater than it would have to be if the reed lifted away from the plate uniformly. And, again, greater reed lift height is also associated with noise and reed stress. If flow is too limited by the valve, especially in the case of a discharge valve, an excessive so called overshoot pressure can occur, meaning the degree to which pressure in front of the piston exceeds the elevated pressure created in the compressor discharge cavity. Overshoot pressure is a good measure of excess, wasted work done by the compressor, and is also the cause of compressor vibration, shaking and noise, which can be a greater problem than inherent reed noise.
Reed valves of the type described may also be used as suction valves, that is, the valves which open on piston back stroke so as to allow low pressure refrigerant vapor to enter the cylinder bore, to then be compressed on the forward stroke. Suction valve reeds do not have metal stop members, however, since the piston head generally comes too close to the back surface of the plate. However, the suction valve may also be a disk shaped, thin metal piece carried on and with the front end of the piston itself, rather than an elongated reed.
Every valve must have a port to be covered and uncovered by the reed, of course, and these are typically round only, because of the obvious ease with which a round hold can be drilled or simply punched through. Non round or circular ports are disclosed in existing prior art patents, however, although not generally found in production compressors. At least one patent, U.S. Pat. No. 4,257,457 issued Mar. 24, 1981 shows a non circular valve port that is elongated with an apparently constant width, with what might be referred to as a "stadium" rather than an ovoid shape. The port is also covered by a unique reed that is axially slidable up and down at both ends, rather than bending up about a single hinge point.
The prior art also shows non circular valve ports with an ovoid or trapezoidal shape, that is, wider at one end than at the other. However, the known prior discloses the narrower end of the port at or near the unconfined free end of the reed, where the opening or "lift" height is greater, not vice versa. For example, U.S. Pat. No. 4,781,540 issued Nov. 1, 1988 to Ikeda, et al. shows, in one embodiment, an ovoid port so oriented, with the narrower end at the free end of the reed. However, there appears to be no discussion of that aspect of the port shape or its orientation, and the real point of the invention is to situate the port, be it circular or non circular, asymmetrically relative to the axis reed. The same patent also discloses non circular port shapes other than ovoid, including kidney shapes. The stated objective of the patent is to cause the reed to open and close with a sideways bend, rather than flat to the plate, so as to prevent the development of oscillatory vibrations in the reed.
Another non circular port shape, disclosed in U.S. Pat. No. 4,778,360, issued Oct. 18, 1988 to Ikeda et al. is essentially the polygonal equivalent of an ovoid, that is, trapezoidal, and the patent is very definite in orienting the narrow end of the port at the free, higher lift end of the reed. It is claimed that placing the wider end of the port beneath the lower lifting main body of the reed, and the narrower end of the port beneath the higher lifting free end of the reed, will yield a more constant flow rate and pressure. That is, where the reed lift is lower, the port is wider, and vice versa. Since this patent has the same assignee and lead inventor as the '540 patent described just above, it is logical to assume that the same philosophy was behind the identical orientation of the ovoid ports there, although that particular claimed advantage was not articulated.
While not prior art per se, co assigned and co pending U.S. patent application U.S. Ser. No. 08/416,123, allowed Aug. 29, 1995 now U.S. Pat. No. 5,672,053, a compressor reed valve with a circular port was disclosed, but with an elongated, recessed channel behind and opening into the port. While the channel itself is itself substantially straight and constant in width, it blends into the circular port with rounded edges which give the circular port and channel together a somewhat elliptical outline, with the wider end of the outline near and beneath the free end of the port. However, the port itself, which is the only part of the outline that actually pierces through the plate, is disclosed as circular only, with no explicit teaching that it be anything but circular or round.